1. Technical Field of the Invention
The present invention relates to the field of integrated circuits, and more particularly to mask-programmable read-only memory (MPROM).
2. Prior Arts
Mask-programmable read-only memory refers to those types of memories into which data are written during the manufacturing process, more particularly through pattern transfer. Among all kinds of mask-programmable read-only memories, three-dimensional mask-programmable read-only memory (3D-MPROM) has an extremely large capacity and low cost.
U.S. Pat. No. 5,835,396, issued to Zhang on Nov. 10, 1998, describes a typical 3D-MPROM. As illustrated in FIG. 1A, a 3D-MPROM typically comprises a semiconductor substrate 0s and a 3D-MPROM stack 0, which is stacked above the substrate 0s. Transistors are built in the semiconductor substrate 0s using standard technology. This 3D-MPROM stack 0 comprises two vertically stacked memory levels (ML): 100 and 200. They are laid down parallel to the substrate 0s, i.e. in the x-y plane and the memory level 200 is stacked above the memory level 100, i.e. along the z direction. Each memory level (e.g. 100) comprises a plurality of address-selection lines (e.g. 20a, 20b . . . ; 30a, 30b . . . ) and memory cells (e.g. 1aa, 1ab, 1bb, 1ba . . . ). Contact vias (e.g. 20av) couple memory levels (e.g. 100) to the transistors in the substrate 0s. 
In prior arts, 3D-MPROM is binary, i.e. its memory cells have two states and each cell can store a single bit. FIG. 1B is the top view of two prior-art binary 3D-MPROM cells 1ba, 1bb, and FIG. 1C is its cross-sectional view. Each cell (e.g. 1ba) comprises an upper address-selection line (ASL) (e.g. 20b) and a lower ASL (e.g. 30b). An insulating dielectric 16 separates the upper ASL from the lower ASL. Existence or absence of an opening in the insulating dielectric 16 determines the information stored in the cell. At cell 1bb, because no opening exists in the insulating dielectric, no current flows during read and this cell represents logic “0”. On the other hand, at cell 1ba, because there exists a full-opening 3ba, a read current flows during read and this cell represents logic “1”. Note this full-opening exposes the whole overlapping area between the upper ASL 20b and the lower ASL 30a. Because it stores single-bit-per-cell, the prior-art binary 3D-MPROM has a limited storage density. It is highly desirable for an 3D-MPROM to store multi-bit-per-cell. Accordingly, the present invention discloses an N-ary (N>2) three-dimensional mask-programmable read-only memory (N-3DMPROM).